SAN FRANCISCO (AP) - Facebook CEO Mark Zuckerberg says he is donating nearly US$500 million (S$609 million) in stock to a Silicon Valley charity to focus on health and education issues. The gift of 18 million Facebook shares is valued at US$498.8 million based on their closing stock price on Tuesday, when Mr Zuckerberg, 28, made the donation. The beneficiary is the Silicon Valley Community Foundation, a nonprofit that works with donors to allocate their gifts. This is Mr Zuckerberg's largest donation to date. He pledged US$100 million in Facebook stock to Newark, New Jersey, public schools in 2010, before his company went public earlier this year. Later in 2010, he joined Giving Pledge, an effort led by Microsoft founder Bill Gates and investor Warren Buffett to get the richest Americans to donate most of their wealth.